1. Field of the Invention
The present invention relates to a laminated ceramic capacitor, and in particular, to an improvement for raising the dielectric constant and electrical insulation property of a dielectric ceramic forming a dielectric ceramic layer provided in a laminated ceramic capacitor.
2. Description of the Related Art
One effective method for satisfying requirements of miniaturization and large capacity in a laminated ceramic capacitor is thinning the dielectric ceramic layers provided in the laminated ceramic capacitor. Nowadays, a dielectric ceramic layer is thinned to as small as less than 1 μm thick.
However, as the thinning of a dielectric ceramic layer progresses, it becomes difficult to ensure the electrical insulation property. Since the insulation property decreases as the number of crystal grains between internal electrodes in the dielectric ceramic layer decreases, it is necessary to decrease grain sizes of the crystal grains to some extent. On the other hand, when the grain sizes of the crystal grains are decreased, the dielectric constant of the dielectric ceramic also decreases.
Therefore, achievement of both a high dielectric constant and high insulation property is a major problem. For achieving both of these, the following techniques are known in a thin layer article in a laminated ceramic capacitor.
Japanese Unexamined Patent Publication No. 2005-243890 discloses an arrangement where crystal grains of dielectric ceramic are composed of first crystal grains having relatively large grain sizes and second crystal grains having relatively small grain sizes, the mean grain size of the first crystal grains is 0.1 to 0.3 μm, and the grain sizes of the first crystal grains are five times or more of the grain sizes of the second crystal grains. It is also disclosed that the areal percentage of the first crystal grains in one observation face of the dielectric ceramic layer is preferably 71 to 91%.
Japanese Unexamined Patent Publication No. 2005-33070 discloses an arrangement where crystal grains of dielectric ceramic include first crystal grains having relatively small diameters, and second crystal grains having larger grain sizes, and the areal percentage of the second crystal grains is smaller than 40%.
As to the thickness of a dielectric ceramic layer, both Japanese Unexamined Patent Publication No. 2005-243890 and Japanese Unexamined Patent Publication No. 2005-33070 disclose “1.5 μm or less”, and in particular, Japanese Unexamined Patent Publication No. 2005-243890 discloses “around 1.0 μm”. However, in the art described in Japanese Unexamined Patent Publication No. 2005-243890 and Japanese Unexamined Patent Publication No. 2005-33070, when the thickness of a dielectric ceramic layer is less than 1 μm, it is impossible to achieve both a high dielectric constant and high electrical insulation property.